1. Field of the Invention
The present invention relates to a driving method for an ink ejection device.
2. Description of Related Art
Of non-impact type printing devices which have recently taken the place of conventional impact type printing devices and have greatly propagated in the market, ink-ejecting type printing devices have been known as being operated on the simplest principle and as being effectively used to easily perform multi-gradation and coloration. Of these devices, a drop-on-demand type for ejecting only ink droplets which are used for printing has rapidly propagated because of its excellent ejection efficiency and low running cost.
The drop-on-demand types are representatively known as a Kyser type, as disclosed in U.S. Pat. No. 3,946,398, or as a thermal ejecting type, as disclosed in U.S. Pat. No. 4,723,129. The former, or Kyser type, is difficult to design in a compact size. The latter, the thermal ejecting type requires the ink to have a heat-resistance property because the ink is heated at a high temperature. Accordingly, these devices have significant problems.
A shear mode type printer, as disclosed in U.S. Pat. No. 4,879,568, has been proposed as a new type to simultaneously solve the above disadvantages.
As shown in FIGS. 1(a) and 1(b), the shear mode type of ink ejection device 600 comprises a bottom wall 601, a ceiling wall 602 and a shear mode actuator wall 603 therebetween. The actuator wall 603 comprises a lower wall 607 which is adhesively attached to the bottom wall 601. An upper wall 605 is adhesively attached to the ceiling wall 602 and polarized in the direction indicated by an arrow 609. A pair of actuator walls 603 thus formed forms an ink channel 613 therebetween. A space 615 which is narrower than the ink channel 613 is also formed between neighboring pairs of actuator walls 603 in an alternating relationship to the ink channels 613.
A nozzle plate 617, having nozzles 618 formed therein, is fixedly secured to one end of each ink channel 613, and electrodes 619 and 621 are provided as metallized layers on both side surface of each actuator wall 603. Each of the electrodes 619,621 is covered by an insulating layer (not shown) to insulate it from the ink. The electrodes 621 which face the space 615 are connected to ground 623, and the electrodes 619 which are provided in the ink channel 613 are connected to a silicon chip which forms an actuator driving circuit 625.
Next, a manufacturing method for the ink ejection device 600 as described above will be described. First, a piezoelectric ceramic layer, which is polarized in a direction as indicated by an arrow 611, is adhesively attached to the bottom wall 601 and a piezoelectric ceramic layer, which is polarized in a direction as indicated by an arrow 609, is adhesively attached to the ceiling wall 602. The thickness of each piezoelectric ceramic layer is equal to the height of each of the lower walls 607 and the upper walls 605. Subsequently, parallel grooves are formed on the piezoelectric ceramic layers by a diamond cutting disc or the like to form the lower walls 607 and the upper walls 605. Further, the electrodes 619 are formed on the side surface of the lower walls 607 by a vacuum-deposition method, and the insulating layer, as described above is provided onto the electrodes 619. Likewise, the electrodes 621 are provided on the side surfaces of the upper walls 605 and the insulating layer is further provided on the electrodes 621.
The vertex portions of the upper walls 605 and the lower walls 607 are adhesively attached to one another to form the ink channels 613 and the spaces 615. Subsequently, the nozzle plate 617, having the nozzles 618 formed therein, is adhesively attached to one end of the ink channels 613 and the spaces 615 so that the nozzles 618 face the ink channels 613. The other end of the ink channels 613 and the spaces 615 is connected to the actuator driving circuit 625 and the ground 623.
A voltage is applied to the electrodes 619, 921 of each ink channel 613 from the actuator driving circuit 625, whereby each actuator wall 603 suffers a piezoelectric shear mode deflection in such a direction that the volume of each ink channel 613 increases. For example, as shown in FIG. 2, when a voltage V is applied to the electrodes of the ink channel 613c, an electric field is generated in the actuator wall 603e in the direction indicated by arrows 627 and an electric field is generated in the actuator wall 603f in the direction indicated by arrows 629. Because the electric field directions 627, 629 are at right angles to the polarization direction 609, 611, the actuator walls 603d, 603e deform outward to increase the volume of the ink channel 613c by the piezoelectric shear effect, resulting in a decrease in the pressure in the ink chamber 613c, including near the nozzle 618c. The negative pressure is maintained for a duration of time T corresponding to a duration of time during which time pressure wave propagates one way of the ink channel 613. During the time duration T, ink is supplied from a manifold (not shown). At this time, the meniscus 24 retracts toward the interior of the ink channel 613c as shown in FIG. 3(b). The duration of time T is necessary for a pressure wave to propagate across the lengthwise direction of the ink channel.
The duration of time T is given by L/a wherein L is the length of the ink channel 613 and a is the speed of sound through the ink filling channel 613. Theories on pressure wave propagation teach that at the moment the duration of time L/a elapses after the rising edge of voltage, the pressure in the ink channel 613 inverts to a positive pressure. The voltage applied to the electrode 619c of the ink channel 613c is returned to 0 V in synchronization with the timing when the pressure in the ink channel 613 is inverted so that the actuator walls 603e, 603f revert to their initial shape shown in FIG. 1(a). The pressure generated when the actuator walls 603e, 603f return to their initial shape is added to the inverted positive pressure so that a relatively high pressure is generated in the ink channel 613c. This relatively high pressure ejects an ink droplet 26 from the nozzle 618c as shown in FIG. 3(c) through 3(g).
However, a portion of the relatively high pressure is consumed in pushing the meniscus 24 toward the aperture of the nozzle 618c to return the meniscus 24 to the shape shown in FIG. 3(a). This wasted portion of the pressure does not contribute to ejection of the ink droplet. The remaining pressure may be insufficient to eject a sufficiently large ink droplet, thereby resulting in poor print quality.
A co-pending U.S. patent application Ser. No. 08/393,391 filed Feb. 23, 1995 by Qiming Zhang and assigned to the same assignee of the present application proposes increasing volume of the ink channel to generate pressure wave in the ink channel, and upon expiration of a predetermined period of time defined as approximately the time interval multiplied by an odd number equal to or greater than three, decreasing volume of the ink channel. By such a driving control, a part of the volume of ink in the ink channel can be expelled outwardly of the nozzle prior to ejecting the ink droplet and hence a relatively large volume of ink droplet can be ejected.
If no succeeding pulses are applied to the electrodes 619e, 619f of the ink channel 613c after ink ejection, the pressure in the ink channel 613c continues fluctuating for a while at a cycle of 2T. These are the residual pressure fluctuations. When the frequency of the voltage pulses is changed, the ink ejection speed changes due to the residual pressure variations, so that the ink droplet arrival point is shifted and thus print quality is degraded. At worst, the printing cannot be performed.
Japanese Patent Laid-Open Publication (Kokai) No. SHO-62-299343 describes applying a cancel pulse subsequent to application of a print pulse for ejection of ink so that the residual pressure fluctuations in the ink chambers are decreased. After a predetermined duration of time elapses following ejection of ink, a cancel pulse is applied for generating a pressure wave with a phase that is exactly opposite to the phase of residual pressure fluctuations in the ink chamber. For example, as shown in FIG. 5(a), a cancel pulse D with a width 3T and a phase that is opposite to the phase of the ejection pulse is applied to the electrode 619 of the ink channel 613 after expiration of a time T following the falling edge of the ejection pulse C. The voltage level of the cancel pulse D is determined to just cancel out the residual pressure fluctuations depending on the amplitude of the fluctuations, e.g., 0.6 times as large as the amplitude of the ejection pulse. By application of the cancel pulse, the actuator wall 603 deforms in the direction opposite from the direction it deformed for ejecting ink. A pressure wave with phase that is opposite the phase of the residual pressure fluctuations is applied to cancel out the residual pressure fluctuations. Application of the cancel pulse eliminates the fluctuations of the ink ejection speed when the frequency of the voltage pulses is changed. As a result, an excellent print quality can be obtained.
The similar advantages can be obtained when a cancel pulse D with a width T and a phase that is opposite the phase of the ejection pulse is applied to the electrode 619 after expiration of time T following the falling edge of the ejection pulse C as shown in FIG. 5(b).
Next, a drive circuitry for cancelling residual pressure fluctuations will be described. The output signals X, Y, and Z shown in FIG. 7 are for applying voltages V, 0, and -0.6 V respectively to the electrode 619 in the ink channel 613. When the output signal X is at a high level, voltage pulses for ejecting ink are generated (pulses C shown in FIG. 5(a)). When the output signal Z is at a high level, a voltage pulse for causing cancellation of pressure fluctuations is generated (pulse D in FIG. 5(a)). In all other circumstances, the output signal Y is at a high level so that 0 voltage is output. Capacitors 91 are formed from the actuator wall 603 of the ink channel 613 and the electrodes 615, 619 formed to the both sides of the actuation wall 603.
The drive circuitry is formed from the three blocks surrounded by broken lines. Each block includes an injection charge circuit 82, a discharge circuit 84, and a cancellation pressure generation circuit 86. A high level input signal X renders the transistor Tc conductive so that a positive voltage V from the positive power source 87 is applied to the electrode E of the capacitor 91 via a resistor R12. A high level input signal Y renders the transistor Tg conductive so that electrode E of the capacitor 91 is grounded via the resistor R12. A high level input signal Z renders the transistor Ts conductive so that a negative voltage -0.6 V from a negative power source 88 is applied to the capacitor 91 via the resistor R12.
The residual pressure variations can also be canceled out by applying a cancel pulse E as shown in FIGS. 7(a) and 7(b). The cancel pulse E is in phase with the ejection pulse C and has a width T and a voltage level determined dependent on the amplitude of the residual pressure fluctuation (for example, 0.5 times as large as the voltage level of the ejection pulse). The cancel pulse E is applied after expiration of time 2T following the falling edge of the ejection pulse C. A driving circuitry for achieving such a cancellation does not require the negative power source 88 as shown in FIG. 6. However, in addition to the positive power source 87 for generating the voltage V, another positive power source for generating a voltage 0.5 V needs to be provided, and these two positive power sources are switched over so as to be selectively used.
According to the driving methods as described, the residual pressure fluctuations are canceled after the pressure wave generated by the application of the ejection pulse C, that is a positive voltage V, propagates one reciprocal way or one and a half of the reciprocal way through the ink channel 613. Therefore, either a cancel pulse D with a negative voltage or a cancel pulse with a positive voltage but differing in the voltage level needs to be applied. This requires a combination of a positive power source and a negative power source or a combination of two positive power sources supplying different voltage levels. The configuration of the driving circuitry is thus complicated and the manufacture thereof is costly.